Lead-acid batteries include a plurality of electrode plates. The plates are arranged to establish alternating positive and negative electrodes. A separator is disposed between each pair of electrodes. The separators are formed of insulating material and prevent metallic deposits in the battery from forming short circuits between the electrode plates. The separator is porous, however, to the battery electrolyte so that current can pass from one plate to another.
Hydrolysis of water at the positive electrode of the battery produces oxygen. The oxygen reacts with the lead of the negative electrode to produce lead oxide. The lead oxide is thereafter reduced to metallic lead, liberating the oxygen which then reforms water. Generally, an increase in the rate with which the just described oxygen recombination process proceeds increases the operating voltage of the battery. The oxygen recombination process is limited by the rate of oxygen transport from the positive electrode to the negative electrode.
Amorphous silica chains have a surface chemistry that facilitates oxygen transport by a phenomena known as absorption and exchange. Accordingly, when silica is disposed between battery electrodes, the surface atoms of the silica readily bond to either oxygen or hydroxyl groups. The production of oxygen at the positive electrode and the depletion of the oxygen at the negative electrode establishes an oxygen concentration gradient that drives the transport of oxygen along the silica chains from the positive to the negative electrode.
After silica particles are mixed with the sulfuric acid electrolyte of the battery, the silica forms a three dimensional reticulated structure throughout the electrolyte. The reticulated silica increases the viscosity of the electrolyte to such an extent that the resultant fluid is commonly described as a gel.
Oxygen transport in a lead-acid battery also takes place in the form of diffusion of the gaseous phase of oxygen through open channels between the electrodes. The just described silica-based electrolytic gel provides, in addition to oxygen transport via absorption and exchange, fissures or channels through which gaseous oxygen can move between electrodes. The channels are formed in the gel as a result of a small amount of drying and cracking of the gel as water is lost during initial operation of the battery.
In the past, separators formed of micro-fibrous borosilicate glass mats or felt have been employed as battery separators. These separators (commonly known as absorptive glass mat separators) generally comprise a blend of glass fibers of varying length and diameter. The capillarity of the mats retains the electrolyte within the separator. Preferably, the mat is designed to saturate to about 80% to 95% with fluid electrolyte so that a void volume exists within the separator. This void volume provides gas channels through which the oxygen that is generated at the positive electrode may travel to the negative electrode. Accordingly, absorptive glass mat separators provide a high rate of oxygen transport.
Complex equipment is needed for placing and compressing the separator between the plates so that the appropriate contact will be developed. Moreover, absorptive glass mats have very low stiffness and are prone to tearing. Therefore, they have poor handling characteristics.